JP6668019B2 - Electrode catalyst for fuel cell, fuel cell, and method for producing the electrode catalyst - Google Patents

Description

  The present invention relates to an electrode catalyst used for a fuel cell, a fuel cell using the electrode catalyst, and a method for producing the electrode catalyst.

  From the viewpoint of depletion of fossil fuels and air pollution caused by their use, fuel cells have been attracting attention as new energy without using fossil fuels, and have actually begun to be used.

  Among them, polymer electrolyte fuel cells (PEFCs) are expected to be used for home on-site power generation and fuel cell vehicles because of their low operating temperatures. However, polymer electrolyte fuel cells have not spread as expected because of their high price. This is because platinum (Pt), which is likely to be depleted, is used as an electrode catalyst for the oxygen reduction reaction (ORR) on the air electrode side of the polymer electrolyte fuel cell.

  Further, a phosphoric acid fuel cell (PAFC) has a relatively low operating temperature, and is currently widely used for industrial large-scale power generation and the like. However, also in the phosphoric acid type fuel cell, Pt is used as the electrode catalyst on the air electrode side similarly to the polymer electrolyte fuel cell, so there is room for improvement from the viewpoint of economy.

  On the air electrode side of the polymer electrolyte fuel cell and the phosphoric acid fuel cell described above, protons from the electrolyte membrane and electrons from the conducting wire react with oxygen to cause an oxygen reduction reaction. However, the efficiency of this oxygen reduction reaction is very low. Therefore, a Pt catalyst has been used for the purpose of sufficiently promoting such a reaction (for example, Patent Document 1). One of the reasons why Pt is suitable for the electrode catalyst material of the oxygen reduction reaction is that its electronic structure is suitable, and more specifically, the strength of adsorption and desorption of oxygen on Pt is high. It is preferable to other metals (for example, Non-Patent Documents 1 to 3).

  Another reason that Pt is suitable as an electrode catalyst material for an oxygen reduction reaction is that Pt has durability against an acid in an electrolyte membrane constituting a fuel cell.

  Pyrochlore-type oxides have been reported as a substitute for Pt in the above-described fuel cell electrode catalysts (for example, Patent Document 2). However, such pyrochlore type oxides are not sufficient to be used as electrode catalysts from the viewpoint of low acid resistance and low durability.

  As described above, the electrode catalyst of a fuel cell represented by a polymer electrolyte fuel cell, a phosphoric acid fuel cell, or the like, which has sufficient catalytic activity as a material other than Pt and has durability against acid The development of a catalyst material was desired.

JP-A-5-36418 Japanese Patent No. 5684971

V. Stamenkovic, B .; S. Mun, K .; J. J. Mayrhofer, P .; N. Ross, N.M. M. Markovic, J .; Rossmeisl, J .; Greenley, and J.M. K. Nφrskov, Angew. Chem. Int. Ed. 2006, 45, 2897-2901. J. Zhang, M .; B. Vukmilovic, Y .; Xu, M .; Mavrikakis, and R.M. R. Adzic, Angew. Chem. Int. Ed. 2005, 44, 2132-2135. F. H. B. Lima, J .; Zhang, M .; H. Shao, K .; Sasaki, M .; B. Vukmilovic, E .; A. Ticianelli, and R.S. R. Adzic, J .; Phys. Chem. C 2007, 111, 404-410.

  In view of the above problems, the present invention has a high oxygen reduction ability as an air electrode side electrode catalyst material in a fuel cell such as a polymer electrolyte fuel cell or a phosphoric acid fuel cell in which protons pass through an electrolyte membrane. Another object of the present invention is to provide an electrode catalyst material other than a noble metal, which exhibits excellent catalytic activity and has excellent durability against an acid, and a method for producing the electrode catalyst material.

  The present inventors have intensively studied to solve the above-mentioned problem. As a result, the electronic structure of the boride or carbide of one or more transition metals selected from the group consisting of cobalt (Co) and nickel (Ni) is similar to the electronic structure of Pt, which easily desorbs oxygen, and The present inventors have found that the catalyst exhibits sufficient catalytic activity as a catalyst and has excellent durability against an acid, thereby completing the present invention. Specifically, the present invention provides the following.

  (1) The present invention is an electrode catalyst used for a fuel cell in which protons permeate an electrolyte membrane, and includes catalyst particles supported on a carrier, wherein the catalyst particles include cobalt (Co) or nickel (Ni). A) a boride or carbide of one or more transition metals selected from the group consisting of:

  (2) The present invention is the electrode catalyst according to (1), wherein the particle size of the catalyst particles is 50 nm or less.

(3) Further, according to the present invention, in a perchloric acid aqueous solution having a pH of 1.01 , a reaction start potential of an oxygen reduction reaction measured by linear sweep voltammetry using a rotating electrode is 0.4 V or more ( An electrode catalyst according to 1) or (2).

  (4) Further, the present invention is a fuel cell in which protons containing the electrode catalyst according to any one of (1) to (3) permeate an electrolyte membrane.

  (5) The present invention also relates to a method for producing an electrode catalyst used for a fuel cell in which protons permeate an electrolyte membrane, wherein at least one selected from the group consisting of cobalt (Co) and nickel (Ni). In an aliphatic amine solution containing a transition metal, a carrier is dispersed, a precursor solution preparing step of preparing a precursor solution, and a heating step of heating the precursor solution under a gas atmosphere containing oxygen. It is a manufacturing method.

  (6) In the present invention, the carrier includes carbon particles, and a mass ratio of the transition metal contained in the precursor liquid to a mass of carbon atoms contained in the carrier is 0.1 or more and 2 or less ( 5) A method for producing an electrode catalyst according to 5).

  (7) The present invention is the method for producing an electrode catalyst according to (5) or (6), wherein the concentration of the metal contained in the aliphatic amine solution is from 5 mmol / L to 50 mmol / L.

  (8) The present invention is the method for producing an electrode catalyst according to any one of (5) to (7), wherein the heating temperature in the heating step is from 250 ° C. to 350 ° C.

  (9) The present invention is the method for producing an electrode catalyst according to any one of (5) to (8), wherein the heating time in the heating step is more than 2 hours.

  ADVANTAGE OF THE INVENTION According to the electrode catalyst which concerns on this invention, it shows high catalyst activity, it also has the outstanding durability with respect to an acid, and can be suitably used as an electrode catalyst of a fuel cell etc. of the form in which protons permeate an electrolyte membrane. it can.

FIG. 3 is a diagram illustrating a configuration of a fuel cell in which protons pass through an electrolyte membrane. FIG. 4 is a flowchart for explaining a method for producing an electrode catalyst. FIG. 4 is a TEM photograph of a Co ₂ C electrode catalyst obtained under the conditions of a heating temperature of 320 ° C. and a heating time of 24 hours in Examples. In an Example, it is a figure which shows the XRD pattern of the powder obtained at the heating temperature of 290 degreeC. FIG. 4 is a view showing an XRD pattern of a powder obtained at a heating temperature of 320 ° C. in an example. It is a figure showing the result of oxygen reduction performance evaluation of the Co ₂ C electrode catalyst obtained in the example. In Example illustrates the repeated evaluation test of 2000 times, the results of durability evaluation of the oxygen reduction performance of Co 2 _C electrocatalyst.

  Hereinafter, a specific embodiment of the present invention (hereinafter, referred to as “the present embodiment”) will be described in detail. Note that the present invention is not limited to the following embodiments at all, and can be implemented with appropriate modifications within the scope of the present invention.

≪1. Electrode catalyst≫
The electrode catalyst according to the present embodiment is an electrode catalyst used for a fuel cell in which protons pass through an electrolyte membrane, such as a polymer electrolyte fuel cell and a phosphoric acid fuel cell. Specifically, the electrocatalyst includes boride or carbide particles of one or more transition metals selected from the group consisting of cobalt (Co) and nickel (Ni) supported on a carrier. In the electrode catalyst according to the present embodiment, the above-described particles are dispersed on the surface using carbon particles or the like as a carrier.

[Catalyst particles]
As described above, the catalyst particles are borides or carbides of one or more metals selected from the group consisting of cobalt (Co) and nickel (Ni). Hereinafter, particles of one or more transition metal borides or carbides selected from the group consisting of cobalt (Co) and nickel (Ni) constituting the electrode catalyst are referred to as “catalyst particles”.

  Further, as the catalyst particles, the main component of the metal contained in the particles may be the above-mentioned metal, and other elements may be doped.

  The particle size of the catalyst particles is not particularly limited, but is preferably 100 nm or less, more preferably 50 nm or less, further preferably 20 nm or less, and particularly preferably 10 nm or less. Since the catalyst particles having a particle size in such a range have a large specific surface area, higher catalyst activity can be exhibited.

  The average particle size of the particles can be measured by TEM observation. Specifically, the average particle diameter of the particles can be determined, for example, by measuring the particle diameters of 100 particles randomly selected from a TEM photograph and calculating the average value of the particle diameters.

[Carrier]
The carrier is such that catalyst particles exhibiting catalytic activity are dispersed and supported on the surface thereof.

  The carrier is not particularly limited as long as it can support the catalyst particles, but has conductivity, and has proton conductivity as a carrier for supporting an electrode catalyst used in a fuel cell in which protons pass through an electrolyte membrane. Are preferred.

  Among them, it is more preferable to use carbon particles. The carbon particles have conductivity and moderate proton conductivity. Further, since the carbon particles have porosity, the specific surface area can be increased, the catalyst particles can be dispersed in a large amount and uniformly on the surface of the carbon particles, and the catalyst activity can be improved. The electrical characteristics of a fuel cell using the same can be improved.

  Specifically, as the carbon particles, for example, acetylene black, ketjen black, vulcan, black pearl, graphitized acetylene black, graphitized vulcan, graphitized Ketjen black, graphitized black pearl, carbon nanotube, carbon nanofiber, Carbon nanohorn, carbon nanofibril and the like can be mentioned.

  As described above, the carrier is preferably one having proton conductivity, but may be a carrier holding a medium having proton conductivity.

The BET specific surface area of the support such as carbon particles is not particularly limited, but is preferably 50 m ² / g or more, more preferably 100 m ² / g or more, and further preferably 200 m ² / g or more. preferable. By using a carrier having such a BET specific surface area, the catalyst particles can be dispersed in a large amount and more uniformly on the surface of the carrier.

[Catalyst particles supported on a carrier]
As described above, the electrode catalyst according to the present embodiment is configured by supporting catalyst particles on a carrier such as carbon particles. The ratio between the carrier and the catalyst particles in this electrode catalyst is not particularly limited, but is preferably, for example, about carrier: catalyst particles = 50: 50 to 30:70.

  The electrode catalyst according to the present embodiment exhibits good activity as an electrode catalyst used in a fuel cell in which protons such as a polymer electrolyte fuel cell permeate an electrolyte membrane, particularly as an electrode catalyst on the air electrode side, On the air electrode side of the fuel cell, the reaction efficiency of the oxygen reduction reaction between the protons, the electrons from the conducting wire, and the supplied oxygen can be increased.

Specifically, initiation of the reaction potential in the oxygen reduction reaction of catalyst particles is not particularly limited, in the over-chlorate in an aqueous solution having a pH of 1.01, using a rotating electrode, the linear sweep voltammetry (LSV) The reaction initiation potential of the measured oxygen reduction reaction is 0.4 V or higher, preferably 0.5 V or higher, and more preferably 0.6 V or higher.

  Here, the reaction starting potential of Pt, which has been conventionally used as an electrode catalyst, is about 0.91 V. Compared with the catalytic activity of Pt, the activity of the above-described catalyst particles is slightly inferior. By increasing the number of cells in the fuel cell using the catalyst particles supported on the carrier, it is possible to obtain a fuel cell exhibiting performance equal to or higher than that of a fuel cell using Pt as an electrode catalyst.

  Further, the above-described catalyst particles have excellent durability against acids. Therefore, it can be suitably used as an electrode catalyst used in a fuel cell such as a polymer electrolyte fuel cell in which protons pass through the electrolyte membrane, and high catalytic activity can be maintained for a long time.

{2. Fuel cell≫
The fuel cell according to the present embodiment is a fuel cell in which protons permeate the electrolyte membrane, and is characterized in that it includes catalyst particles supported on a carrier as an electrode catalyst. And, as described above, the catalyst particles are particles of boride or carbide of one or more metals selected from the group consisting of cobalt (Co) and nickel (Ni).

  FIG. 1 is a schematic diagram illustrating an example of a schematic configuration of a fuel cell. As described above, the fuel cell 1 is a fuel cell in which protons permeate the electrolyte membrane, such as a polymer electrolyte fuel cell (PEFC) and a phosphoric acid fuel cell (PAFC). In the following, the fuel cell 1 will be described as a polymer electrolyte fuel cell.

  As shown in FIG. 1, the fuel cell 1 includes a fuel electrode 11, an electrolyte membrane 12, an air electrode 13, and separators 14a and 14b. In this fuel cell 1, each of the fuel electrode 11 and the air electrode 13 is provided with a gas diffusion layer 15a, 15b and a catalyst layer 16a, 16b. Then, in the fuel cell 1, at least the catalyst layer 16b constituting the air electrode 13 contains catalyst particles supported on a carrier as an electrode catalyst. Note that the electrode catalyst including the catalyst particles may be included in the catalyst layer 16a constituting the fuel electrode 11.

(1) Fuel Electrode The fuel electrode 11 is supplied with hydrogen as a fuel gas, and the hydrogen is oxidized to generate a proton and an electron (e ⁻ ), an oxidation reaction of hydrogen (formula (i) below) occurs. Is the pole of

In the fuel electrode 11, a flow path of a fuel gas containing hydrogen is formed, and a gas diffusion layer 15a for evenly diffusing the fuel gas introduced into the flow path and a gas diffusion layer 15a for promoting an oxidation reaction of hydrogen contained in the fuel gas. And a catalyst layer 16a. Specifically, the fuel gas containing hydrogen introduced into the fuel electrode 11 passes through a flow path defined by a separator 14a described later, is uniformly diffused in the gas diffusion layer 15a, and is sent to the catalyst layer 16a. Then, in the catalyst layer 16a, as described above, an oxidation reaction of hydrogen contained in the fuel gas occurs.
_{^{H 2 → 2H + + 2e -}} ··· (i)

  At the fuel electrode 11, protons generated by the above-described oxidation reaction of hydrogen move to the air electrode 13 side through the electrolyte membrane 12. On the other hand, electrons generated by the oxidation reaction move toward the air electrode 13 via a conductor (conductive wire) connected between the fuel electrode 11 and the air electrode 13.

(2) Electrolyte membrane The electrolyte membrane 12 is provided between the fuel electrode 11 and the air electrode 13. Protons generated in the fuel electrode 11 pass through the electrolyte membrane 12 and move toward the air electrode 13, and reach the catalyst layer 16 b constituting the air electrode 13.

  The electrolyte membrane 12 is not particularly limited as long as it allows protons to pass therethrough. As the PEFC, for example, a cation exchange membrane or the like can be used. In the case of PAFC, a membrane made of phosphoric acid or the like can be used as the electrolyte membrane.

  Further, it is preferable to use a strongly acidic electrolyte membrane 12 because of having a property of transmitting protons. For the same reason, the catalyst layer 16b constituting the air electrode 13 to which protons reach through the electrolyte membrane 12 may be durable to acids and may not cause corrosion or the like. preferable.

(3) Air electrode The air electrode 13 is supplied with air, and oxygen in the air generates water by protons that have moved from the fuel electrode 11 through the electrolyte membrane 12 and electrons that have moved through the conductor. On the side where the oxygen reduction reaction (formula (ii) below) occurs.

The air electrode 13 includes a gas diffusion layer 15b in which air is formed and uniformly diffuses the air introduced into the flow path, and a catalyst layer 16b for promoting a reduction reaction of oxygen in the air. . Specifically, the air introduced into the air electrode 13 passes through a flow path defined by a separator 14b described later, and is sent to the catalyst layer 16b, which is uniformly diffused by the gas diffusion layer 15b. Then, in the catalyst layer 16b, as described above, a reduction reaction occurs by oxygen, protons, and electrons in the air. As described above, the electrons flow from the fuel electrode 11 to the air electrode 13 to generate power.
2H ⁺ + 1 / 2O ₂ + 2e ⁻ → H ₂ O (ii)

  The fuel cell 1 according to the present embodiment is characterized in that at least the catalyst layer 16b constituting the air electrode 13 includes catalyst particles supported on a carrier as an electrode catalyst. The electrode catalyst made of the catalyst particles can effectively increase the reaction efficiency of the reduction reaction between the protons, the electrons from the conducting wire, and the oxygen in the supplied air on the air electrode 13 side.

(4) Separator The separators 14a and 14b are provided around the fuel electrode 11 and the air electrode 12, and partition a fuel gas (reducing agent) and air (oxidizing agent) and form a gas flow path. In the separator 14a on the fuel electrode 11 side, a flow path for introducing a fuel gas into the fuel electrode 11 is formed on the surface on the gas diffusion layer 15a side. On the other hand, in the separator 14b on the air electrode 13 side, a flow path for introducing the air to the air electrode 13 is formed on the surface on the gas diffusion layer 15b side.

{3. Manufacturing method of electrode catalyst≫
As described above, the electrode catalyst according to the present embodiment is used for a fuel cell in which protons pass through the electrolyte membrane, and includes catalyst particles supported on a carrier such as carbon particles. The electrode catalyst can be produced, for example, by passing through a precursor solution preparation step of preparing a precursor solution for producing the electrode catalyst and a heating step of heating the precursor solution under a specific gas atmosphere. .

Hereinafter, a method for producing an electrode catalyst in which cobalt carbide particles (Co ₂ C) as catalyst particles are supported on carbon particles as carriers will be described as an example.

(1) Precursor Solution Preparation Step The precursor solution preparation step is a step of preparing a precursor solution by dispersing carbon particles as a carrier in an aliphatic amine solution containing cobalt.

  Regarding the aliphatic amine solution containing cobalt, the aliphatic amine is not particularly limited as long as it has a relatively high boiling point and a reducing performance.

  Specifically, the carbon number of the aliphatic amine is not particularly limited, but preferably 13 or more, more preferably 14 or more, and still more preferably 15 or more. When the number of carbon atoms of the aliphatic amine is less than 13, the boiling point of the solvent becomes low, so that heating cannot be performed at a high temperature, which may lower the production efficiency.

  Moreover, the carbon chain of the aliphatic amine may have only a saturated bond or may have an unsaturated bond. Further, the carbon chain may be linear or branched, or may be alicyclic.

  Among such aliphatic amines, it is preferable to use oleylamine from the viewpoint of having a high boiling point (348 ° C. to 350 ° C.) and an appropriate reducing property.

  The cobalt source is not particularly limited, but a cobalt salt, a cobalt complex or the like can be used. As the cobalt salt, cobalt chloride, cobalt bromide, cobalt iodide, cobalt acetate, cobalt nitrate, cobalt sulfate and the like can be used.

Further, as the cobalt complex, those whose ligands are, for example, aliphatic carboxylic acids, aromatic carboxylic acids, amines, aminopolycarboxylic acids, amino acids, β-diketones and the like can be used. Among them, the cobalt complex is preferably cobalt acetylacetonate from the viewpoint that small-sized Co ₂ C particles can be effectively obtained.

The concentration of cobalt in the aliphatic amine solution is not particularly limited, but is preferably 5 mmol / L or more, more preferably 6 mmol / L or more, and more preferably 7 mmol / L or more based on the cobalt atom. Is more preferable. If the concentration of cobalt is less than 5 mmol / L, the yield of Co ₂ C particles may decrease. On the other hand, the upper limit of the cobalt concentration is not particularly limited, but is preferably 50 mmol / L or less, more preferably 40 mmol / L or less, and further preferably 20 mmol / L or less. If it exceeds 50 mmol / L, a uniform precursor solution may not be obtained.

  In the precursor solution, the mass ratio of the cobalt atom to the mass of the carbon atom contained in the carbon particles is not particularly limited, but is preferably 0.1 or more, and more preferably 0.25 or more. Is more preferable, and 0.4 or more is more preferable. If the mass ratio is less than 0.1, sufficient catalytic activity may not be obtained. On the other hand, the upper limit of the mass ratio is preferably 2.0 or less, more preferably 1.0 or less, and even more preferably 0.6 or less. If the mass ratio exceeds 2.0, the obtained particles may be aggregated.

  As described above, as the carbon particles, for example, acetylene black, ketjen black, vulcan, black pearl, graphitized acetylene black, graphitized vulcan, graphitized ketjen black, graphitized black pearl, carbon nanotube, carbon nanotube Nanofibers, carbon nanohorns, carbon nanofibrils, and the like can be used.

  The method for dispersing the carbon particles in the solution is not particularly limited, and a general stirring or dispersion technique can be used. Specifically, it can be performed by a stirring method using a stirrer or a shaker, or a dispersion method using an ultrasonic cleaner, an ultrasonic disperser or the like.

  The atmosphere for preparing the precursor solution is not particularly limited, and the preparation can be performed in the air, under an oxygen atmosphere, or under an inert gas atmosphere exemplified by nitrogen or argon.

(2) Heating Step The heating step is a step of heating the precursor liquid prepared in the precursor liquid preparation step in a gas atmosphere containing oxygen. By this heating step, Co ₂ C particles can be uniformly deposited on the surface of the carbon particles.

In this heating step, first, cobalt oxide (CoO) is generated by heating the precursor solution, and CoO is changed to Co ₂ C by continuing heating. That is, in the heating reaction in the heating step, CoO is generated as a so-called precursor, and the crystal becomes Co ₂ C via the precursor.

As a gas atmosphere for heating, heating is performed in a gas atmosphere containing oxygen. Specifically, examples of the gas atmosphere include a mixed gas of oxygen and various inert gases, air, and the like. Among them, an air atmosphere is preferable from the viewpoint of industrial economy. On the other hand, when heating is performed in an atmosphere containing no oxygen, the precursor CoO is not generated, and as a result, no Co ₂ C is generated.

The heating temperature depends on the type of the solvent to be used, and may be appropriately adjusted in consideration of the boiling point of the solvent and the like. Further, the heating temperature is more preferably 260 ° C. or more and 340 ° C. or less, and further preferably adjusted to 270 ° C. or more and 330 ° C. or less. In the present embodiment, since an aliphatic amine which is a high boiling point solvent is used, heating can be performed under such high temperature conditions without applying pressure. If the heating temperature is lower than 250 ° C., the production rate of Co ₂ C particles becomes slow, and efficient production cannot be achieved. On the other hand, if the heating temperature exceeds 350 ° C., the solvent used may be boiled. However, when the precursor solution is pressurized, the temperature can be set to 350 ° C. or higher.

The heating time depends on the heating temperature, and may be appropriately adjusted in consideration of the heating temperature. However, the heating time is preferably longer than 2 hours. The heating time is more preferably 5 hours or more, further preferably 10 hours or more, and particularly preferably 20 hours or more. If the heating time is less than 2 hours, insufficient heating causes CoO as a precursor to be generated to not change to Co ₂ C, so that a Co ₂ C crystal structure cannot be effectively obtained. There is.

  In addition, heating can be performed using a general reactor.

(3) Treatment Step Although not an essential aspect, the carbon particle-supported Co ₂ C catalyst obtained in the heating step can be subjected to post-treatments such as a washing treatment and a drying treatment (post-treatment step). . A dry powder is obtained by performing washing and drying processes in this post-processing step.

For example, in the post-treatment step, first, the aliphatic amine in which the carbon particle-supported Co ₂ C catalyst obtained in the heating step is dispersed is cooled. Next, the resultant is washed with a solvent to sufficiently remove the aliphatic amine and the like, and then dried to obtain a carbon particle-supported Co ₂ C catalyst.

  The method for cooling the aliphatic amine solvent is not particularly limited, and for example, it can be left as it is and slowly cooled. Further, cooling can be performed using a cooler.

  The washing solvent is not particularly limited, and for example, water, methanol, ethanol, 1-propanol, 2-propanol, acetone, ethyl acetate, hexane and the like can be used. One of these washing solvents can be used alone, or two or more solvents can be used in combination. Further, the cleaning process can be performed a plurality of times. In addition, since aliphatic amines, acetylacetone, trace amounts of cobalt ions, and the like used as raw materials have different solubilities in the solvent, it is preferable to use two or more solvents for washing a plurality of times, and to wash the solvent. More preferably, hexane and ethanol are used as seeds.

  The washing method is not particularly limited, and a general method for washing particles can be used. For example, washing can be performed together with centrifugation and filtration. In a large-scale facility, cleaning can be performed by batch-type cleaning.

  The drying treatment is not particularly limited, and the particles can be dried using a general method for drying particles. For example, the drying can be performed using a drying method such as natural drying, heated drying, reduced pressure drying, and freeze drying.

  The manufacturing method shown as an example as described above is a method for manufacturing an electrode catalyst using Co2C particles as catalyst particles, but, of course, when synthesizing an electrode catalyst using catalyst particles made of other metals. Can also be applied.

  For example, when nickel carbide is used as the catalyst particles, it can be manufactured using a nickel salt or a nickel complex as the nickel source instead of the cobalt source in the above-described manufacturing method.

Further, in one example of the above-described production method, carbon particles as a carrier are used as a carbon source of Co ₂ C. However, when a carbon-free material is used as a carrier, other Can be added and used.

  Similarly, when a boride or a nitride is produced, a carrier may be used or an additive other than the carrier may be added and used as a boron source or a nitrogen source.

  Hereinafter, the present invention will be described more specifically with reference to Examples, but the present invention is not limited to the following Examples.

In this embodiment, cobalt carbide (Co ₂ C) particles are selected as an example of the catalyst particles, and the electrode catalyst in which the cobalt carbide particles are supported on carbon particles as a carrier is as follows. , (2) analysis of the properties of the cobalt carbide catalyst, (3) evaluation of the oxygen reduction performance of the cobalt carbide catalyst, and (4) durability evaluation of the cobalt carbide catalyst by a repeated oxygen reduction test.

(1) Preparation of Carbon Black-Supported Cobalt Carbide Catalyst (Co ₂ C / C) FIG. 2 shows a flowchart of a method for preparing a carbon carbide catalyst (Co ₂ C / C) supported on carbon black. First, a precursor of cobalt was introduced into oleylamine (a chemical formula [NH ₂ (CH ₂ ) ₈ (CH) ₂ (CH ₂ ) ₇ CH ₃ ] manufactured by Tokyo Chemical Industry Co., Ltd.), a formula amount of 267.5, which is a high boiling point solvent. cobalt acetylacetonate dihydrate (abbreviated _{Co (acac) 2 · 2H 2} O, the chemical formula _{[Co (C 5 H 7 O 2} ) 2 ] · _2H 2 O, formula weight 293.18) as a body of 147mg additives Then, 50 mg of carbon black (Vulcan carbon XC-72 (surface area 250 m ² / g, manufactured by CABOT)) as a carrier was added and dispersed (Co / C = 50 wt%).

  Next, the heating temperature was set to 290 ° C. or 320 ° C., and heating was performed at each temperature for 30 minutes to 24 hours.

  After the elapse of the heating time, the heating and stirring were stopped, and the flask was sufficiently cooled. Then, 50 ml of hexane was added to each content and centrifuged. The remaining solid content excluding the liquid phase was further dispersed well with 50 ml of ethanol and centrifuged. This ethanol washing was performed twice in total. The solid thus obtained was dried at a temperature of 50 ° C. in the atmosphere.

(2) Analysis of Properties of Prototype Catalyst FIG. 3 shows a transmission electron microscope (TEM) photograph of the produced powder obtained under the conditions of a heating temperature of 320 ° C. and a heating time of 24 hours. As shown in the TEM photograph of FIG. 3, relatively small dark particles (small particles) of 10 nm or less were observed on the carbon black carrier which appeared relatively pale. As a result of identifying these small particles by energy dispersive X-ray spectroscopy (EDX), peaks derived from Co were observed, and peaks derived from other elements were not observed. From the result of such TEM observation, it was found that the obtained particles were dispersed on the carbon support, the particle size was about 10 nm, and Co was contained.

Next, the obtained product powder was subjected to powder X-ray diffraction (XRD) measurement. FIG. 4 shows carbon obtained by heating at a heating temperature of 290 ° C. for (a) 30 minutes, (b) 1 hour, (c) 2 hours, (d) 12 hours, and (e) 24 hours. The XRD measurement result of the black supported cobalt carbide catalyst (Co ₂ C / C) is shown. FIG. 5 shows the results obtained by heating at a heating temperature of 320 ° C. for (a) 30 minutes, (b) 2 hours, (c) 5 hours, (d) 12 hours, and (e) 24 hours. 4 shows XRD measurement results of a carbon black-supported cobalt carbide catalyst (Co ₂ C / C). In the XRD measurement, a Cu-Kα ray was used, and a diffraction pattern was observed under a general condition of 40 kV-30 mA and 2θ of 30 ° to 80 °, and a crystal structure analysis was performed.

As a result of XRD measurement, the crystal structure of the particles obtained by heating for 24 hours at any temperature was consistent with the Co ₂ C structure. From these results, it was found that the formation of Co ₂ C was accelerated by increasing the heating temperature.

Further, it was found from the results of the respective heating times that the crystal structure of Co ₂ C was formed via the crystal structure of CoO. Regarding this point, when the synthesis was performed under the condition that the CoO structure was not formed by performing this experiment in a nitrogen atmosphere, the Co ₂ C structure was not formed. This also indicates that the Co ₂ C crystal structure is formed after the CoO crystal structure is formed.

On the other hand, the molar ratio (Co: C) of cobalt atoms and carbon atoms in Co ₂ C obtained under the conditions of a heating temperature of 320 ° C. and a heating time of 24 hours was measured by X-ray photoelectron spectroscopy (apparatus: manufactured by Kratos Analytical). (ESCA 3400), it was Co: C = 68: 32.

(3) Evaluation of oxygen reduction performance of cobalt carbide catalyst In order to measure the ORR activity of Co ₂ C / C nanoparticles obtained at a heating temperature of 320 ° C. and a heating time of 24 hours, a perchloric acid (HClO ₄ ) aqueous solution was used. Using a rotating electrode in the inside, the reaction initiation potential in the oxygen reduction reaction was measured by linear sweep voltammetry (LSV).

  In addition, such a method is a general one among electrochemical analysis methods, in which a potential applied to a system to be measured is changed, a current that changes in response to the change is measured, and the current is analyzed. This is a method for performing analysis. That is, the higher the reduction potential, the more easily the reduction reaction proceeds.

The evaluation was performed using a rotating ring disk electrode device (RRDE-3A) manufactured by BAS. In this apparatus, a platinum electrode was used as a counter electrode, and an Ag / AgCl electrode was used as a reference electrode. Further, the sweep potential was -0.1 V to 1.2 V, the sweep scan speed was 5 mVs- ¹ and the electrode rotation speed was 1600 rpm. Further, pH of perchloric acid (HClO ₄₎ water solution was 1.01.

Specifically, the formed carbon black-supported cobalt carbide catalyst (Co ₂ C / C) was added to 1-propanol and well dispersed using an ultrasonic cleaner to prepare a 4 mg / ml dispersion. Of these, 5 μl was placed in a sample rotating electrode and air-dried in air. Next, 9 ml of 1-propanol was added to 1 ml of a 20% Nafion (registered trademark) solution (a propanol solution of a sulfonated tetrafluoroethylene copolymer) made of a proton conductor electrolyte material manufactured by Wako Pure Chemical Industries, and mixed. And air-dried in air.

FIG. 6 shows the evaluation results of the oxygen reduction performance. As a control experiment, the same oxygen reduction property was measured for CoO / C and Co / C nanoparticles. As a result, with respect to CoO / C and Co / C nanoparticles, the reaction initiation potentials were 0.25 V and 0.31 V, respectively, and there was no significant difference from the reaction initiation potential of 0.28 V when measured only with the carbon carrier, which was effective. No activity was shown. On the other hand, the Co ₂ C / C nanoparticles had a reaction initiation potential of 0.61 V, indicating a high oxygen reduction reaction activity.

(4) Durability Evaluation of Cobalt Carbide Catalyst by Repeated Oxygen Reduction Test Next, the durability of the formed carbon black-supported cobalt carbide (Co ₂ C / C) nanoparticles as an oxygen reduction reaction catalyst was evaluated.

  As an evaluation method, the potential sweep in the evaluation test described in the preceding section was repeated 2,000 times, the reduction potential at that time was measured, and the change in reduction onset potential before and after the 2000 evaluation tests was evaluated. FIG. 7 shows the evaluation results of the durability.

As shown in FIG. 7, when the starting potentials of the first measurement and the 2000th measurement were compared, the starting potentials were both 0.61 V, and no decrease in the starting potential due to repeated use was confirmed, indicating that the durability was high. Do you get it. Oxygen reduction reaction activity evaluation by the LSV, despite was performed under strongly acidic conditions below pH = 1.01, since the Co 2 _C showed a high oxidation-reduction active without dissolving, Co ₂ C was found to be a highly acid-resistant electrocatalyst.

  As described above, the boride or carbide nanoparticles of one or more transition metals selected from the group consisting of cobalt (Co) and nickel (Ni) show high oxygen reduction reaction activity and can be used in a protic acid environment. It was found to have durability. Then, by using these boride or carbide nanoparticles as an electrode catalyst of an air electrode of a fuel cell in which protons permeate an electrolyte membrane such as a polymer electrolyte fuel cell, the use of platinum is high without using platinum. It was found that a fuel cell having oxygen reduction reaction activity was obtained.

Claims (8)

An electrode catalyst used for a catalyst layer constituting a positive electrode of a fuel cell in which protons permeate an electrolyte membrane,
Including catalyst particles supported on a carrier,
An electrocatalyst, wherein the catalyst particles include a boride of one or more transition metals selected from the group consisting of cobalt (Co) and nickel (Ni) , or a carbide of cobalt (Co) . The electrode catalyst according to claim 1, wherein the particle size of the catalyst particles is 50 nm or less. The electrode catalyst according to claim 1 or 2, wherein the reaction start potential of the oxygen reduction reaction is 0.4 V or more as measured by linear sweep voltammetry using a rotating electrode in a perchloric acid aqueous solution having a pH of 1.01. . A fuel cell comprising the electrode catalyst according to any one of claims 1 to 3 in a catalyst layer constituting a positive electrode , wherein protons pass through an electrolyte membrane. A method for producing an electrode catalyst used for a fuel cell in which protons pass through an electrolyte membrane,
A precursor solution preparing step of preparing a precursor solution by dispersing a carrier in an aliphatic amine solution containing one or more transition metals selected from the group consisting of cobalt (Co) and nickel (Ni);
A heating step of heating the precursor solution at 250 ° C. or higher and 350 ° C. or lower in a gas atmosphere containing oxygen;
A method for producing an electrode catalyst comprising: The production of the electrode catalyst according to claim 5, wherein the carrier includes carbon particles, and a mass ratio of the transition metal included in the precursor solution to a mass of carbon atoms included in the carrier is 0.1 or more and 2 or less. Method. The method for producing an electrode catalyst according to claim 5 or 6, wherein the concentration of the transition metal contained in the aliphatic amine solution is 5 mmol / L or more and 50 mmol / L or less. The method for producing an electrode catalyst according to any one of claims 5 to 7 , wherein the heating time in the heating step is longer than 2 hours.